The purpose of this report is the investigation, more fully, with quantitative procedures, certain preliminary experiments which suggest that the thymus derived cells involved in the initiation of the humoral immune response are similar or are the same thymus derived cells involved in the initial recognition aspects of cell mediated immunity reactions such as the delayed-type hypersensitive footpad swelling reaction of sensitized mice. This will be accomplished with the aid of an in vitro short term culture technique which allows the measurement of T-cell cooperation activity, an activity measured by the enhancement of an in vitro primary antihapten immune response. Limiting dilution techniques will allow the establishment of a minimum number of units responsible for such immune responses. By manipulation of the antigenic stimulation of mice, it will be possible to establish some parameters of the "lymphocyte traffic" of functional immune cells in the mouse and will allow some understanding of the critical parameters involved in the balance between the initiation of the humoral immune response and the expression of cell mediated immunities against certain antigens.